This invention relates generally to audio frequency amplifier circuits used as pre-amplifier stages of microphone transducers. More particularly, the invention relates to circuits employing transconductance devices operating with low voltage supplies and having predominantly capacitive input impedance.
An amplifier for amplifying a signal from a high impedance source, such as an electret or crystal microphone, or other transducers based on piezoelectric principles, requires a high input impedance to avoid impedance mismatches that will attenuate the input signal to the amplifier. The reason for this attenuation is the voltage division that occurs between the transducer's source impedance and the input impedance of the amplifier. Representing the source impedance as z.sub.s, the input impedance as z.sub.in, the voltage source v.sub.s, and the input voltage to the amplifier v.sub.in, the input voltage to the amplifier is given by the known relationship: ##EQU1## If the impedances are characterized as purely capacitive, as is the case with an electret microphone connected with a transconductance device such as a JFET, equation 1 becomes ##EQU2## where the transducer capacitance is c.sub.s and the amplifier input capacitance is c.sub.in. It is obvious from (2) that as c.sub.in is made smaller relative to c.sub.s, there will be less attenuation of the source voltage v.sub.s at the input to the amplifier, v.sub.in. This will have the beneficial effect of a larger signal to noise ratio, and a greater output signal.
In hearing-aid technology, the use of electret acoustic transducers is common. An electret acoustic transducer has a very low signal output voltage, a very high output impedance which is predominately capacitive, and a very broad output frequency spectrum. In addition, in hearing-aid technology a single cell low voltage power supply is used for amplification purposes. The use of a single cell low voltage power supply, typically 1.3 volts or so, means that often the desired signal at the preamplifier input is only slightly higher in voltage than the undesired noise. At the output it is very important that an amplifier be able to make use of the entire available signal voltage in order to achieve a suitable signal-to-noise-ratio. Noise from electronic components and feedback signals from other amplifier stages coupled through the low impedance power source make it imperative that the entire output voltage signal from an electret acoustic transducer be used in order to achieve a suitable signal to noise ratio.
In hearing aid technology it is usual for high input impedance transconductance devices to be used as pre-amplifier stages. These devices are typically used in a follower configuration allowing the output impedance to be at a lower value suitable for following amplifier stages with higher gain and lower input impedance. It is common for FET transistors to be used for this purpose. Various methods have been utilized to achieve this desired result.
Prior art on electret-based condenser microphone pre-amplifiers most commonly describes circuits in which a single cell is used and the common reference point for input and output is the lower potential rail. This allows the use of N channel junction field effect transistors, generally the quietest of the field effect transistors, for the active element. Although the following description uses circuits with this orientation, it should be understood that the invention is not limited to this orientation, but can equally apply to circuits where the upper rail is the ground reference or where a split supply is used.
Also, the following description is not limited to the JFET sub-class of FETs, but applies to FETs in general. N channel JFETs are, however, the preferred embodiment.
Early electret-based condenser microphones for hearing aid application had transducer elements with 7 to 10 pfd. source capacitance. The amplifier following the microphone would often have a low input impedance, in the kilo-ohm (K.OMEGA.) or tens of K.OMEGA. range. Because of this mismatch in impedance, it was found necessary to employ a field effect transistor (FET) as a buffer between the transducer and the main amplifier so as to minimize signal loss. For these early electret-based condenser microphones, such a transistor was considered acceptable if it had an input capacitance and transconductance such that signal attenuation at both the input and output was minimal.
For such microphones prior art generally employed a single junction FET (JFET), most commonly a 2N4338 or a manufacture specific equivalent as the transistor and applied it in the source follower mode so as to minimize the effective input capacitance and output impedance. For the follower mode of operation, it is well known that c.sub.gs, the gate to source capacitance inherent in the transistor, is reduced by the open circuit gain of the stage, which can be made large enough so as to make the effective gate to source capacitance negligible.
In prior art, a very large resistor, often in the G.OMEGA. (giga-ohm) range was used for gate bias. Later art such as U.S. Pat. No. 3,512,100 to Killion et al and U.S. Pat. No. 4,151,480 to Carlson et al modified the JFET follower circuitry by the replacement of the gate bias resistor with a parallel pair of opposing polarity diodes. This was done because the real part of the impedance presented by the diodes was much larger than could be normally attained by a passive, screened on, resistive element. In general, the higher the real part of the impedance for the bias element, the lower the noise introduced by the bias element. Resistors screened on ceramic, if properly manufactured, are almost purely resistive elements. Unfortunately, the proper manufacture of giga-ohm (G.OMEGA.) sized resistors requires that they be very long compared to low valued resistors. If kept physically short the a.c. impedance, although still resistive may be a small fraction of the d.c. impedance. Apparently there is some capacitive shunting along the length of the resistor, which reduces its effective impedance without adding a capacitive term to impedance measured across the terminals. Diodes, on the other hand, are formed by the junction of two semiconductor materials of opposing polarity and inherently form a capacitive element as part of their structure. This capacitance is effectively a parallel element to the "ideal" diode. As c.sub.gd, the gate to drain capacitance of the JFET, is inherent in the gate to drain diode junction of the JFET, it is not surprising that the junction capacitance of the diode pair can be of a similar magnitude as c.sub.gd. Therefore, as shown by equation (2), for the case where the junction capacitance of the diode pair equals c.sub.gd, the signal would be attenuated by about 6 dB more than if the diodes were not present. The use of the diodes has other limitations. As the voltage across a forward biased diode increases, the impedance of the diode decreases, dropping many orders of magnitude until it may be as low as hundreds of ohms. Signal distortion will then occur as the impedance changes. For some applications, studio microphones for example, very high signal levels may be common at times, and such distortion is then a problem. Prior art such as Madaffari et al U.S. Pat. No. 5,097,224 teaches that a G.OMEGA. resistor in series with the diodes will overcome the overload problem, because even when the diodes drop to a low a.c. impedance, the G.OMEGA. resistor will have a high enough impedance compared to the source impedance that distortion will not occur.
U.S. Pat. No. 5,083,095 issued on Jan. 21, 1992 to Madaffari discloses an amplifier utilizing multiple source follower amplifiers. These amplifiers exhibit an improved input impedance; however, the amplifiers of that patent suffer from a reduction in DC operating range making it difficult to operate the amplifiers with low voltage and especially single cell power supplies, with the attendant difficulties described above.
U.S. Pat. No. 5,337,011 issued on Aug. 9, 1994 to French et al., discloses an amplifier utilizing a multiple source follower amplifier with a bias network which includes a pair of oppositely oriented, parallel connected diodes in the bias network. In addition to the reduced DC operating range discussed above, there is no manner of dealing with the junction capacitances of the diode biasing network. This additional capacitance will lead to attenuation of the input signal to the amplifier as described in equations (1) and (2 ) above.